ATSC (Advanced Television Standards Committee) Digital TV, as mandated for broadcast use by the FCC, requires specialized signal processing in the television receiver modulator which is not amenable to low-cost implementation in consumer electronics. This requirement has so far prevented the introduction of low-cost modulators in digital television products to provide the same functionality as the ubiquitous NTSC (National Television Standards Committee) modulators in analog NTSC TV products.
The technology of advanced television systems using digital television signal processing, including the processing of high-definition television (HDTV) signals for consumer broadcast, reception, and presentation, incorporates many advanced signal-processing concepts, methods, and devices. The field's technology addresses the general problem of transmitting high-definition and rapid-motion video streams via both wireless and other modes of communication to large numbers of users concurrently at reasonable cost. The standards for advanced television systems are published by the Advanced Television Standards Committee (ATSC), and are available at www.atsc.org/standards.html. The ATSC standard of particular interest with respect to the present invention is the ATSC Digital Television Standard A/53, incorporated herein by reference.
Also of particular relevance to the present invention is the MPEG-2 standard used to encode moving pictures and associated audio information. The MPEG-2 standard is in continuous evolution—copies of the standard, identified as ISO/IEC 13818, may be purchased at the International Standards Organization (ISO) at the ISO Website: www.iso.org/iso/iso_catalogue/, also incorporated herein by reference.
As defined by the ATSC standard in the present art, the modulation of the MPEG-2 transport stream carrying the information for the viewer is performed using an 8-VSB (8-bit vestigial sideband) modulator. The 8-VSB modulator converts the MPEG-2 stream into a radio-frequency signal to be broadcast or otherwise conveyed to the user's receiver for demodulation, decoding, and presentation as images and sound. An informative article on the 8-VSB modulation process is titled “What Exactly is 8-VSB Anyway?”, by David Sparano, available on the Web at http://www.broadcast.net/˜sbe1/8vsb/8vsb.htm and incorporated herein by reference.
The modulation process as described by ATSC Standard: Digital Television Standard (A/53), Revision D, 19 Jul. 2005 consists of the following steps:                1. The MPEG-2 transport stream is processed as described in the standard cited hereinabove and applied to an 8-VSB modulator. In the present art, the digital processing required prior to the 8-VSB modulator is readily accomplished by integrated circuits and does not create a significant cost or complexity problem.        2. The resulting channel stream applied to the S-VSB modulator is post-filtered by a raised-cosine-squared Nyquist filter to confine the signal to the allocated 6 MHz-wide channel, and to form a matched filter in combination with the second raised-cosine-squared filter in the television receiver. Because of the severe restrictions placed by the FCC on radiation in adjacent channels this filtering is sophisticated, and is typically done at low intermediate frequencies (IFs) such as 45 MHz, where thermal drift of the components is not a problem in maintaining the exacting filter profile.        3. The resulting IF ATSC signal must then be frequency-converted to the desired channel, and the resulting undesired image mixing product suppressed by filtering where it falls in the TV band(s), either directly or by subsequent conversions.        4. Allowing the modulator to be channel-agile, meaning that the modulator can process signals acceptably and uniformly for output on any channel, normally requires additional complexity, typically by conversion of the IF ATSC signal first to a 2nd high IF above the TV bands, so that a second conversion to a selectable final channel may be done using a low-pass filter to suppress 2nd IF feed-through        5. The pilot carrier frequency in the final channel is initially positioned to center the signal's upper sideband in the 6-MHz channel, and may be modified by exacting offsets under certain conditions of adjacent or co-channel usage. This combination of precision and agility requires elaborate frequency synthesis.        
The modulator constructed in accordance with the above steps is both too large and too expensive to replace NTSC modulators in consumer equipment as it is migrated to support ATSC digital TV.
For additional background, two articles, one titled “A Compatible Narrowband 8VSB Transmission System”, published by Axcera of Lawrence, Pa., available at http://broadcast.axcera.com/bet_paper.pdf, and the other titled “Architecture of a DSP Based Dual-Mode ATSC/NTSC Television Exciter and Transmitter”, by David L. Hershberger, Continental Electronics, Inc., available at http://www.contelec.com/pdf%5Cdspdtv.pdf, are incorporated herein by reference.
A good general reference on digital signal processing is the book titled “Understanding Digital Signal Processing” by Richard G. Lyons, Addison Wesley Longman 1997, ISBN 0-201-63467-8.